The present invention relates to an antenna device in which an FM antenna for receiving FM broadcast and a planar antenna for receiving GPS signals and/or satellite digital radio broadcast, and so on are incorporated.
In an antenna device installed on a vehicle, antennas for receiving FM broadcast, satellite digital broadcast and/or GPS signals are respectively required. However, in case where these antennas are separately arranged, an outer appearance would be unattractive, and for this reason, a plurality of antennas are incorporated in a single casing of such antenna device. Moreover, as the antenna device, a compact and low profile antenna device is desirable. Therefore, use of a top capacity loaded type monopole antenna is considered in place of a rod antenna. Examples using such top capacity loaded type monopole antenna are disclosed in JP-A-2010-21856 and JP-A-2009-135741. In the antenna device disclosed in JP-A-2010-21856, both an FM antenna and a planar antenna for receiving GPS signals are incorporated. In the antenna device disclosed in JP-A-2009-135741, an FM antenna and a planar antenna for receiving satellite digital broadcast are incorporated.
In the art disclosed in JP-A-2010-21856, a capacity loading plate of the top capacity loaded type monopole antenna to be used as the FM antenna is arranged at a position separated upward from an antenna base plate and in parallel with a flat surface of the antenna base plate. Therefore, it is possible to realize an antenna device having a relatively low profile. However, in case where a planar antenna is disposed on the antenna base plate and below the capacity loading plate, it is predicted that gain of the planar antenna would be deteriorated due to influence of the capacity loading plate.
Moreover, in the art disclosed in JP-A-2009-135741, a monopole antenna which is used as the FM antenna is uprightly provided on an antenna base plate. Therefore, it is predicted that a planar antenna disposed on the antenna base plate and below the FM antenna would be unlikely to be influenced by the FM antenna. However, because the FM antenna is uprightly provided with respect to the antenna base plate, it is impossible to realize the antenna device having a low profile.
The inventors of the invention arranged, as a first step, a capacity loading plate of the top capacity loaded type monopole antenna to be used as the FM antenna, at a position separated upward from an antenna base plate and in parallel with a flat surface of the antenna base plate, and disposed a planar antenna on the antenna base plate and below the capacity loading plate, in the same manner as in the art disclosed in JP-A-2010-21856. Then, the inventors carried out measurements for confirming what extent the gain of the planar antenna would be deteriorated due to the influence of the capacity loading plate. FIG. 14 is a perspective view of a simulation model for measuring the gain of a GPS patch antenna which is disposed below the capacity loading plate. A GPS patch antenna 12 which is the planar antenna is disposed at a center of a ground plane 10, and a capacity loading plate 14 of a top capacity loaded type monopole antenna is arranged above the GPS patch antenna 12, at a position of a height H of 50 mm. A length L of this capacity loading plate 14 is 100 mm, and a width W thereof is 30 mm. FIG. 15 shows changes in the gain of the GPS patch antenna 12, in the structure as shown in FIG. 14, in case where a center of the capacity loading plate 14 is displaced from a center of the GPS patch antenna 12 in directions of width and length. The GPS patch antenna 12 has again of 7 dBic in a direction of zenith, unless the capacity loading plate 14 is provided. The gain decreases to 3 to 4 dBic, when the GPS antenna 12 is displaced from the center of the capacity loading plate 14 in a range of about ±25 mm in the direction of width and about ±30 mm in the direction of length, and the gain is relatively decreased. The gain decreases to 4 to 5 dBic, when the GPS antenna 12 is displaced from the center of the capacity loading plate 14 in a range of about ±45 mm in the direction of width and about ±50 mm in the direction of length, and the gain is remarkably decreased. It is possible to reduce such influence of the capacity loading plate 14, by remarkably displacing the center of the capacity loading plate 14 from the center of the GPS patch antenna 12. However, in this case, an area for installing the antenna device is increased.
Under the circumstances, it is desired that the antenna device is made low profiled by arranging the capacity loading plate of the top capacity loaded type monopole antenna to be used as the FM antenna, at the position separated upward from the antenna base plate and in parallel with the flat surface of the antenna base plate, as in the art disclosed in JP-A-2010-21856, and moreover, the planar antenna disposed on the antenna base plate and below the capacity loading plate is not influenced by the capacity loading plate.